1. Field of the Invention
This invention relates to a spring assembly, which is used as, for example, a return spring member for a multi-plate clutch piston in a clutch mechanism of an automatic transmission of a motor vehicle, and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, this kind of spring assembly, although not shown, includes two annular plates and plural compressing springs arranged at regular intervals in a circumferential direction of the annual plates and fixed to these annular plates. In actual assembling, end turns of the compression coil springs are set to outer peripheries of protrusions formed on an inside surfaces of each annular plate to face to each other. Then, each protrusion is expanded outwardly by using a punch for caulking, so that the outer peripheries of the protrusions and the inside surfaces of the annular plates retain the end turns of the compression coil springs.
With such a configuration, in the process where one end turn of the compression coil spring is attached to one annular plate, the punch for caulking can be easily caused to approach the protrusion. Thus, the protrusion can be caulked relatively easily. On the other hand, in the process where the other end turn of the compression coil spring is attached to the other annular plate cannot be inserted into an area between the annular plates, easily. In this case, a very difficult and troublesome operation is required.
In order to avoid this, a method has been proposed in which when the one end turn of the compression coil spring is attached to the one annular plate, the protrusion is caulked as before by using an ordinary punch for caulking. On the other hand, when the other end turn of the compression coil spring is attached to the other annular plate, the protrusion is caulked by using a longitudinal-dividing punch and diameter-expanding pin.
Specifically, the longitudinal-dividing punch has an outer diameter, which is slightly smaller than an inner diameter of the protrusion. A tip end thereof is divided by defining a division groove to be expandable. The diameter-expanding pin has an outer diameter enough to be inserted into the protrusion of the other annular plate. A sharp tip end thereof is guided into the division groove of the longitudinal-dividing punch, so that the tip end of the longitudinal-dividing punch can be expanded.
While the sharp tip end of the expanding pin is facing the protrusion of the other annular plate to be undergone the caulking operation, the tip end of the longitudinal-dividing punch inserted through the protrusion on the one annular plate, which has already been caulked, is lowered. As a result, the sharp tip end of the expanding pin is guided into the division groove to enlarge the tip end of the longitudinal-dividing punch. Thus, it becomes possible to expand the protrusion outwardly and caulk the protrusion. (see Japanese Patent No. 3288458).
Therefore, in the conventional manufacturing method, the protrusion of the other annular plate can be easily caulked by using the longitudinal-dividing punch and diameter-extending pin. Therefore, the operation of assembling the other annular plate and the compressing coil pin can be improved. However, when the protrusion is caulked, the entire periphery of the protrusion is not uniformly diameter-expanded, but the protrusion is expanded only partially at the tip end of the longitudinal-dividing punch, which is divided by the division grooves. Thus, the retaining force of the compression coil spring on the other annular plate is lowered, so that the compression coil spring might be detached therefrom. Further, since the tip end of the longitudinal-dividing punch itself is divided through the division grooves, the strength of the tip end is low so that the tip end is apt to break or wear.